Electronic device



Julie 8, 1948. D, v, EDWARDS 2,443,100

ELECTRONIC DEVICE Filed March 13. 1945 2 sheets-sheet 1 F le. 1

. c v l *m 1 f L@ l f/'a FIG. 2A

FIG. 2C

M.. Jnnventor l June 8, 1948- D. v. EDWARDS ELECTRONIC DEVICE 2Sheets-Sheet 2 Filed March 13, '1945 FIG. 3

Patented June 8, 1948 UNITED STATES PATENT OFFICE ELECTRONIC DEVICEDonald V. Edwards, Montclair, N. J., assigner to Electrons,Incorporated, Newark, N. J.

Alllldation March 13, 1945, Serial No. 582,487 14 Claims. (Cl. 315-246)This invention relates to electronic devices of the gaseous dischargetype, and more particularly to methods andapparatus for improving thelife and performance of gas discharge rectifiers and grid-controlrectifier tubes in circuit organisations where a high inverse voltage isapplied immediately after conduction through the tube ceases. A

Experience has shown that the life of gas discharge rectifying tubes,and more particularly tubes of the hot cathode type with an inert gasillling, is materially dependent upon the conditions under which aninverse voltage is applied,

as well as the magnitude of such inverse voltage. Among other things, itis found that tube life is adversely affected by a sudden application ofan excessive inverse voltage immediately after conduction ceases, suchadverse aiiect being dependent to a large degree upon the amount ofcurrent being conducted, and the magnitude of the inverse voltage. This'deleterious effect upon tube life is ordinarily manifested by areduction in the normal gas pressure, insumcient electron emission and adefinite increase in the arc drop, with the result that the tube failsmuch sooner than its normal life under other different operatingconditions. This characteristic of gas discharge rectifying tubes isgenerally recognized as imposinga limitation upon their rating forcertain applications and uses.

My investigations have indicated that the adverse ellect o f theapplication of a. high inverse voltage to a tube immediately afterconduction ceases is primarily due to the sputtering of anode materialcaused by the bombardment of the anode with positive ions as they areaccelerated by the electric tl'eld of the inverse voltage. It isbelieved that such sputtering of the anode material tends to trap themolecules of the gas filling on the walls of the tubef thereby reducingthe normal gas pressurean'd it may be this sputtering of anode materialadversely affects the electron emission of the cathodeby what may betermed a, poison-ing action. In any event, sputtering of anode materialCauses"' in time an increase in the resistance of the arc drop, and thetube fails to conduct properly.

This injurious sputtering of the anode material may be explained on thetheory that, during the time the tube -is conducting current, ionizedatoms o! the gaseous filling of the tube are present throughout theconducting path, the density or concentration of these ions varyingsomewhat with the magnitude of current being conducted, but beingrelatively dense for all practical currents. In the ordinary gasdischarge tube, the time of natural deionization for dissipating thesegas ions after conduction ceases, either by the process of diffusion, byrecombination on the walls of the tube and electrodes, or by othereffects, is relatively long, in the order of 1000 microseconds. 'I'histime of deionization is shortened by the electric field of an inversevoltage applied to the anode; but the sudden application of a. highinverse voltage before the deionization is substantially complete hasthe eilect of attracting the existing positive ions in the gas fillingtoward the now negatively charged anode, so that if there are enoughpositive ions present and the voltage accelerating them is high enough,there is an excessive ionic bombardment of the anode which sputter-,soil' the anode material.

In view of these considerations, the primary purpose and object of thislinvention is to devise a method and means for obviating this deleterioussputtering of anode material by the sudden application of a high inversevoltage to a gas discharge rectifying tube immediately after conductionceases.

A further object of the invention is to attain this end in a simple andeicient manner so as not to interfere materially with the eiiiciency andperformance of gas discharge rectifying tubes, and permit such tubes tobe employed effectively in various circuit organizations and for varioususes with a normal life expectancy and without the limitations anddisadvantages heretofore encountered.

Various other objects of the invention, its characteristic features,attributes, and advantages will be in part apparent and in part pointedout as the description progresses.

Generally speaking, and without attempting to define the exact natureand scope of the present invention, it is proposed to regulate orcontrol the rate of rise of the potential difference between the anodeand cathode of a gas discharge tube when an inverse voltage is applied,in such a way as to deionize the gas filling effectively without causinginjurious sputtering of anode material. More specifically, it isproposed to employ a cushioning or softening means in connection withthe tube which limits the rise of potential between the anode andcathode upon the application of an inverse voltage to a degreesuiiicient to avoid the objectionable sputtering of anode material thatwould otherwise result from the direct application of inverse voltage tothe tube. In one specific embodiment of the invention, this moderatingor cushioning means takes the form of a capacitor f suitable capacityand a series resistor connected directly across the anode and cathode ofthe tube.

The accompanying drawings illustrate in a diagrammatic and conventionalmanner certain speciflc forms of apparatus for practicing thisinvention, together with graphs of current and voltage for explanatorypurposes, theparts and circuits being shown more with a view offacilitating an explanation` and understanding of the nature of theinvention, rather than to show in detail the physical embodiments thatwould be preferably employed in practice.

, In these drawings, Fig. 1 illustrates the invention applied to acircuit organization employing full-Wave grid controlled rectiflers forsupplying current to a highly inductive load;

Figs, 2A to 2D inclusive, illustrate explanatory curves or graphs ofcertain current and voltage changes for this circuit organization ofFig. 1;

Fig. 3 illustrates the invention applied to a typical inverter of theparallel type;

Figs. 4A to 4C show current and voltage graphs relating to the paralleltype inverter organization of Fig. 3.

The cushioning or softening principles of this invention may be appliedto gas discharge rectifying tubes in any one of their variousapplications and uses where a high inverse voltage is suddenly appliedimmediately after conduction; but an explanation of the characteristicfeatures and functions of the invention as applied to certain typicalcircuit organizations using gas discharge tubes will serve to make clearthe nature and utility of the invention, and indicate how the sameprinciples and practices may be advantageously adopted for otherrectifying circuit organizations.

Fig. 1 illustrates one such typical application of the invention to afull-wave grid controlled v rectifier organization used for supplyingcurrent to a highly inductive load, such as an electromagnetic device,eld coils of the generator or motor, or the like. In the circuitorganization of Fig. 1, which is illustrated diagrammatically andconventionally, the usual anode transformer I has its two secondarywindings 2 and 21 connected in the usual conventiona1 manner to thecathodes and anodes of two grid controlled gaseous discharge tubes; soas to supply current through a highly inductive load Z to the midtap ofthese secondary windings. The grids of the two tubes '1* and T1 may becontrolled by any suitable phase control device, or some equivalentarrangement, so that the firing point of the tubes may be regulated toprovide the desired load current. The inherent leakage inductance oftheanode transformer is a factor to be considered in the analysisof thecircuit, and for convenience the effective leakage inductance of the twosecondary windings 2 and 21l is shown schematically in Fig. 1 asequivalent lumped inductances L and L1.

The cushioning means of this invention as shown applied to the circuitorganization in Fig. 1 ,comprises a capacitor C and a resistor Rconnected in series directly across the anode and cathode of the tube T,and a similar capacitor C1 and resistor R1 connected across the othertube T1. These capacitors and resistors may be of any one of the wellknown types and constructions, having of course the appropriateinsulating and radiating qualities requisite for the voltages andcurrents involved. The factors determining the values of capacitance andresistance to be preferably employed are more conveniently coilsideredlater after discussing their functions and operating characteristics.

As previously indicated, the deleterious sputtering of anode materialadversely affecting the l-fe of a gas discharge rectifying tube isdependent upon the magnitude ofthe inverse voltage suddenly applied-andthe amount of current being conducted Just before the application ofsuch voltage. Generally speaking, where the current conducted varies dueto the change in firing point ofthe tubes, as in the typical applicationshown in Fig. 1, or due to ordinary variations in load, it isycontemplated that the cushioning or softening means of this inventionwill be de signed to *l satisfy the worst conditions in this regard,although it should be understood that a cushioning or softening actionof a lesser degree will be eii'ective in prolonging the life of thetube, and may be adopted, particularly where the extreme conditions ofinverse voltage and current conduction are likely to exist only during asmall part of the time in the actual perform-- ance of the circuitorganization.

yIn describing the principles and functions of this inventionl and thetheory of operation which is believed to explain the results obtained,it becomes necessary to consider the probable occurrences undertransitory conditions existing during extremely short time intervals;and since the phenomena under consideration are not readily measured,certain theoretical premises and deductions are assumed for the purposeof explaining the results. On the basis of such assumptions, Figs. 2A to2D show graphs or curves of voltage and current changes for certainelements of the circuit organization of Fig. l; but it should beunderstood that these curves are presented for explanatory purposes, andare not intended to illustrate the quantitative relations existing inpractice. More particularly, the time dimensions for certain changes asillustrated in these graphs of Figs. 2A to 2D have been greatlydistorted in order thatl changes occurring in microseconds may be shown.

In describing the functions and purposes of the invention, and thetheory of operation attributed to observed phenomena, it is convenientto ana.- lyze separately the transitory conditions existing duringdifferent half-cycles of the impressed voltage, the complete performancerepresenting a repetition of such transitory conditions.

In the typical circuit organization illustrated in Fig. 1, it is assumedthat the inductance Z of the load will serve to maintain currentconduction through each tube until the next tube is fired, particularlywhere the tubes are fired near the peak ofthe positive half-cycle of thesupply voltage as shown. After a few cycles following the closure of thecircuit, the load current may be considered as assuming a, steady statecondition with a slight ripple; and the current curves I and I1 for thetubes T and T1 as shown in Fig. 2B are illustrative of the conductioncurrents through these tubes under such conditions.

Taking the firing of the tube T at the time indicated at to as aconvenient starting point for the discussion, at that instant thecurrent being conducted by the other tube T1 is decaying at a ratedetermined by the characteristics of the load circuit, but has asubstantial magnitude. even though the phase voltage E1 applied to thisAtube T1 is in the reverse direction and has risen during the negativehalf-cycle nearly to its peak value. In this connection, it may beconsidered that the decay of current through the inductance Z of the Yload and the leakage inductance L1 of the secpotential for the tube Tlsuillcient to maintain ionization and conduction in spite of the reversevoltage of this transformer secondary.

Since the arc drop through the tube T1 still conducting at the time tunder consideration is relatively insigniilcant with respect to thephase voltages contemplated, being in the order of some ten volts forthe typical gas discharge tube, it can be seen that Vthe cathode of thetube T1, and likewise by direct connection the cathode of the other tubeT, has supplied thereto the negative potential of the phase voltage E1less the voltage drop across the arc of the tube T1 and the leakageinductance L1 of the transformer, so that atthe time t underconsideration the voltage across the other tube T has risen toapproximately the sum of the two phase voltages as indicated by thevoltage curve in Fig. 2C.

Considering now the conditions at the critical time t0 when the tube Tres, the firing of this tube causes the voltage between its anode andcathode to drop quickly to the relatively insignificant value of the arcdrop through this tube. At this instant, since the other tube T1 is alsoconducting, and the cathodes of the two tubes are directly connectedtogether, the anode of the other tube T1 is at approximately the samepotential. In other words, at the particular time t0 underconsideration, and for a few microseconds until the tube T1 previously-ilred has stopped conducting, the anodes of the two tubes are atsubstantially the same potential, and the output voltage at theterminals of the secondaries of the anode transformer I is zero, thesame as if the transformer were effectively short-circuited. Thiscondition is represented by the abrupt changes to zero in the voltagewaves indicated at I in Fig. 2A. In this connection, the leakageinductances L and L1 of the transformer control rate of change incurrent during this very brief interval; and it may be considered thateach phase voltage of the transformer is during this interval matchedwithin it by an equal and opposite voltage across its leakageinductances.

The abrupt changes to zero in the phase voltages of the transformer justdescribed is not particularly material to the conditions calling for theapplication of the principles of this invention, but this is anattribute of the organization and is mentioned here because, among otherthings, it is involved in the procedure later described for ascertainingthe desired values for the cushioning capacitors and resistors.

The occurrences following the ring of the tube T at the time tD occursso fast with respect to variations of the supply voltage that, althoughthe explanatory graphs in Figs. 2A to 2D suggest that there is time fora change in such supply voltage, it may be considered that the supplyvoltage is constant during the time interval of commutation underconsideration.

Still referring-to the time indicated at t1 when the tube 'I is fired,the ring of thistube T affords a conducting path for load current, inaddition to the conducting path through the other tube T1; but since thelarge inductance of the load tends to prevent any marked change in totalcurrent, it may be considered that 'as the conduction current throughthe tube T rises, there is an accompanying decrease in the current beingconducted by the other tube T1, as indicated in 6 Fig. 2B, until after avery brief interval of commutation, in the order of some microseconds.conduction through the -tube T1 previously fired has completely ceased.Current through the tube T now conducting rises under the influence ofthe positive half-cycle of its phase voltage E for la time, and thenafter this phase voltage re- "verses, starts to decay, as indicated bythe current curve I in IFig. 2B, until the other tube T1 is again fired,whereupon the same commutation phenomena are repeated.

As previously explained, it has been found that :the life of gasdischarge rectifying tubes is adversely affected by the suddenapplication of a high inverse voltage immediately following conduction;and -considering the nature of the inverse voltage applied to the tubeT1 at the in- 'stant it stops conducting indicated as the time t1, itcan be seen that the phase voltages E and E1 of the anodetransformersecondaries are across the tubes T and T1. Since the tube Tis new conducting, and its arc drop-is relatively small. its cathode hasapplied'thereto a potential of approximately -l-E volts; and since thecathode of -the other tube T1 is at the same potential due to the directconnection, the inverse voltage which is suddenly applied across thetube T1 when this tube stops conducting at the instant indicated at t1,is substantially equal to E+E'1, or approximately twice the phasevoltage of the anode transformer. 'Furthen this high inverse voltagefollows immediately within some 30 microseconds after the time when thetube T1 is conducting approximately full load current, as indicated bythe curve I1 at the time t".

Having discussed in some detail in the interests of clarity theoperating conditions in the` circuit organization of Fig. l which causethe sudden application of a high inverse voltage immediately afterconduction of substantial current, consideration may now be given to theprinciples and practices characterizing the method and apparatus ,ofthis invention for obviating the adverse effect upon tube life whichsuch conditions have heretofore imposed.

As previously pointed out, I have found that the life of the gasdischarge rectifying tube is adversely effected under the conditions ofexcessive inverse voltage under consideration by reason of -thesputtering of anode material by the ionic bombardment of the anode.Accordingly, the primary purpose and objective achieved by thisinvention is to minimize the sputtering of anode material under theseconditions to a degree at least where it is no longer a factor in fixingthe normal life expectance of the tube. In other words, ifthe ionicbombardment of the anode can be regulated or controlled to theappropriate degree in accordance with this invention, the resuit will bethat the tube will fail from other causes characteristic of tubes ofthis type, rather than due to the damaging sputtering of anode material,thereby enabling the tube to be employed in circuit organizationsheretofore considered unsuitable for obtaining satisfactory tube life.

Briefly considering the theoretical considerations apparently involvedin this matter of sputtering of anode material, the gas filling of a gasdischarge tube is ionized during the period of conduction, and whenconduction ceases, an appreciable time is required for deionization ofthe existing gas ions representative of the immediately preceding fow ofcurrent through the tube. vSuch deionization will occur in timenaturally by a recombination of positive gas ions with'free electronswhere they may exist at the walls of the tube or the like; but thisnatural period of deionization may be substantially shortened by theelectric ileld of an inverse voltage applied to the tube. Theapplication of such inverse voltage, however, in addition tofacilitating deionization by repelling free electrons toward cathode andother effects, also'acts to attract the existing positive gas ionstoward the anode, which is now negatively charged by this inversevoltage. Theoretical considerations suggest that all of the gas ionsexisting at the time an inverse voltage is applied are not necessarilyattracted to the negatively charged anode, apparently because some ofthe ions are so close to the walls of the tube that they are notsuiiiciently affected by the electric eld of the inverse voltage to becarried to the anode, and because other ions become deionized forvarious reasons before reaching the anode. Also, it is probable that thedamaging sputtering action of ionic bombardment of the anode isdependent upon the energy acquired by the positive gas ions striking theanode, which is variable dependent upon the accelerating inversevoltage, as well as the ion density4 and number of ions reaching theanode.

In this connection, investigations indicate for -typical tube structureswith gas fillings that inverse voltages in the order of 200 to 300 voltsdo not ordinarily cause sufficient sputtering to be a controlling factorin shortening the life of the tube, as compared with other causes oftube failure. It should be understood that these suggested values of anacceptable inverse voltage are merely illustrative, and are materiallyeffected by the structure and capacity of the tube, nature of the gasfilling, and other factors. It is found, however, that higher voltagescause an ionic bombardment for ordinary density of gas ions, which ismaterialy damaging in the sputtering of anode material and definitelyaffects adversely the normal life expectancy of the tube.

On the basis of this analysis of the causes and conditions of thedamaging effect of suddenly applied inverse voltages, it is proposed inaccordance with this invention to regulate or control automatically therise of the actual potential between the anode and cathode of a. tube asthe inverse voltage is applied thereto, dependent upon and in accordancewith the rate of effective ionization of the gas filling of the tubeunder the existing conditions, in order that, during the process ofdeionization under the influence of the inverse voltage, the relativedensity of the ,positive ions, in association with the rising potentialaccelerating these ions, will not result in an accumulated effect ofionic bombardment of the anode in a sufficient degree to adverselyaffect the life of the tube.

In practicing the invention, a suitable agency or means is employed toretard, cushion or soften the rate of rise of the actual potentialbetween the cathode and anode of the tube as the high inverse voltage isapplied; and for this purpose I preferably employ a suitable capacitorand resistor in series, or equivalent electrical elements to absorbtemporarily the applied inverse voltage, so to speak, and restrain therise in the actual potential across the electrodes of the tubes to thedesired rate.

Referring to the typical application of the invention shown in Fig. 1,the cushioning capacitors C and C1 and their associated resistors R andR1 are directly connected across the cathode and anode of the respectivetubes T and T1, and perform the desired purpose of retarding the rise ofpotential across the electrodes of these tubes upon application of theinverse voltage inthe manner now to be explained.

Considering the action of the softening capacitors C and C1 more indetail, and referring to the period prior to the firing of the tube T atthe time t, while the anode potential for the tube T is rising toapproximately the value of E+E1 as indicated by the curve 5 in Fig. 2C,this tube T is non-conducting; and this rise in voltage results incharging the capacitor C as indicated at 6 in Fig. 2D. When the tube Tfires at the time t0, the capacitor C discharges through the tube T inseries with its resistor R, as indicated at 1 in Fig. 2D. The initialvalue of this discharge current ils represented by the voltage ofapproximately E+E1 on the capacitor C, divided by the resistance of theresistor R; and one function of the resistor R is to limit the intensityof this discharge current through the tube T when it lires, becausewithout the resistor R such discharge current though momentary might beso large as to be injurious to the tube. 'I'he discharge current of thecapacitor C dies out according to the usual exponential functiondetermined by the time constant of the circuit afforded by the capacitorC and resistor R, so that while the tube T is firing, the capacitor C issubstantially discharged.

Referring now to the critical time t0 when the tube T fires, the othercapacitor C1 associated with the other tube T1 is substantiallydischarged, as indicated at 8 in Fig. 2D, because this other tube T hasbeen conducting for some time and has afforded a discharge path of lowresistance for its capacitor C1. Consequently, at the critical time t1when conduction through the tube T1 ceases, and the excessive inversevoltage of E+E1 is applied thereto, its associated cushioning capacitorC1 is in a discharged state. This capacitor C1 and its associatedresistor R1 are directly connected to the anode and cathode of the tubeT1; and it can be readily seen that, regardless of the magnitude of theinverse voltage E+E1, the actual potential across the anode and cathodeof the tube T1 cannot exceed the voltage drop across the capacitor C1and its resistor R1. In other words, the rate of rise of potentialapplied to the electrodes of tube T1 is dependent upon the growth of thecharging current to the cushioning or softening capacitor C1, because itis the growth of this current which increases the voltage drop acrossthe resistor R1 and accumulates a charge on the plates of the capacitorC1 to provide the potential across these electrodes.

For these reasons, when the softening means of the invention isemployed, the potential across the cathode and anode of the tube T1 whenit stops conducting at the time t1, instead of jumping abruptly to thefull inverse voltage of approximately E+E1, as indicated by a dashvertical line at I0 in Fig. 2C, is controlled or restrained by thecushioning action of the capacitor C1 and resistor R1 to rise at aslower retarded rate, such as indicated by the line I I in Fig. 2C.

It can be readily appreciated that the growth of charging current to thecushioning capacitors C and C1 may be made as gradual as desired toprovide such rate of increase in potential applied across tube elementsas is necessary for normal life expectance of the particular tubeemployed and circuit organization involved. In this vconnection. lthecircuit path for charging the capacitor C1 at the time t1 when the tubeT1 stops conducting may be traced from the upper terminal of thesecondary 2 of the anode ytrans-- former, leakage inductance L, throughthe arc drop of the tube T now conducting. resistor R1, capacitor C1 andleakage inductance L1 to the lower terminal of the other secondary 21 ofthe anode transformer. It can thus be seen that the rate of increase inthe charging current for the capacitor C1 is dependent upon the resistorR1, as well as the inherent leakage inductances L and L1 of the anodetransformer. Thus, the resistor R1. in addition to limiting the rate ofdischarge of capacitor C1 when the tube T1 lires, also performsthefunction oi" controlling the rate of charging of the capacitor C1. andhence the rate of potential rise between the cathode and anode of thistube T1.

.The quantitative values of the cushioning capacitors C and C1 and theresistors R and R1 preferably employed in practice are dependent upon a.number of factors, including the structural characteristics of thetube, and kind of gas filling, as well as the voltages and currents in'-volved in the particular circuit organization in which the tube is used.The acceptable rate of potential rise between anode and cathode of agiven tube immediately following conduction of substantially full loadcurrent may be estimated by calculation for different tubes when certainfactors for the type of tube are known from observation and test; andlife tests or any other desired procedure, may be employed to cheek theacceptable rate of potential rise between anode and cathode for a giventube structure. Generally speaking. it is found that rates less thanabout 10,000,000 volts per second are acceptable foithe types of gasdischarge tubes which I have investigated; but it should be of courseunder stood that conditions may well exist where either higher or lowerrates of rise of inverse voltages are suitable for adequate tube life.

Once the tolerable rate of inverse voltage rise has been determined fora given tube StIuCtuIe iii-some suitable manner, a desirable value ofthe cushioning capacitors and resistors for any given application ofthis tube in a circuit organization may be determined by calculation, orpreferably by directobservation of the anode voltage trace with anoscilloscope. been previously noted that, during the commutation periodbetween t and t1 in Figs. 2A to 2D. the phase voltages of the anodetransformer drop to zero. as indicated at 4 in Fig. 2A, and thenrecover. If the vertical deflection plates of an osc illoscope areconnected directly across the anodes of tubes T and T1 in theorganization of Fig. l, there can be seen on the voltage trace a similarsudden drop and recovery of the potential during the period ofcommutation; and if the time scale is suitably expanded by operating thesweep circuit of the oscilloscope at a frequency which is a largemultiple of the supply frequency, this change in the trace of the anodevoltage may be enlarged suiliciently to permit measurement, after duecalibration, of therate of rise of the anode potential relative to thecathode. In other Words-'by utilizing the appropriate oscilloscopetechnique, the actual rate of rise of the cathode to anode potentialwith a given value of cushioning capacitor C and resistor R, may beobserved, and appropriate changes made in the values of this capacitorand resistor to bring such observed rate of potential rise within thetoler- In this connection, it has' able limits prescribed for the tubein question. In this connection, as typical or representative of thegeneral order of acceptable'values of cushioning capacitors andresistors, it has been found that a capacitance of one microfarad with aresistance of 200 ohms is suitable for one type of gas discharge tubeemployed in the circuit organization shown in Fig. 1.

With regard to the preferred values of the resistors Rand R1, it can beseen that the addition of a capacitor C -infa circuit organization inthe manner contemplated in accordance with this invention may cause thecircuit to approach an oscillatory condition; andthe values of theresistors R and R1 should be chosen to avoid oscillation, which can bereadily detected by watching for any over-shoot in the trace of anodevoltage on an oscilloscope. In the interest of efficiency and avoidinguseless losses in the resistors R and R1, I consider it preferable toemploy a resistance for these resistors comparable with the value forcritical damping; and generally speaking, it is considered preferable touse resistors R. and R1 corresponding in value tothe square root 0f 4times the sum of the leakage inductances L and L1 in henries, divided bythe capacitance of the capacitors C or C1 in farads.

It should be understood that the theoretical considerations andquantitative relationships explained in the foregoing discussion of theinvention are presented merely to facilitate an understanding of thenature of the invention and how to practice the invention, and that themethod and apparatus of this invention are in nowise limited by thisexplanation, and may be practiced in various other ways in variousdegrees to prolong the life of gas discharge tubes in various circuitorganizations. to any extent desired.

Fig. 3 illustrates the invention applied to a circuit organization for atypical parallel type inverter for converting direct current toalternating current; and Figs. 4A-4C illustrate certain curves or graphsof voltage and current relations for explanatory purposes. In thecircuit organization of Fig. 3. the grids of the gas discharge tubes Tand T1 are controlled in a suitable manner so as to be fired alternatelyat the desired frequency to cause current from a suitable source ofdirect current indicated conventionally at 2li to oW alternately throughthe primaries 2| and 2i1 of an output transformer 22, and therebyprovide an output of alternating current in the secondary of thistransformer. As conventionally illustrated, the grids of the tubes T andT1 are assumed to be biased by a suitable source 23 to prevent firing ofthese tubes until this bias is removed by an impulse voltage from thesecondaries of a grid control transformer 2l, which is governed by asuitable grid control means shown diagrammatically. The usual protectiveresistances 25 are preferably included in the-grid circuit.

The usual commutating or control condenser CC, characteristic of thistype of inverter, is shown connected across the primaries 2|, 2|1 of theoutput transformer 22; and to permit the cushioning means of thisinvention to function to the best advantage. additional inductances XLand XL1 are preferably included in the circuit connections between thiscondenser CC and the anodes of the tubes T and T1. If desired, thecommutating condenser may be connected across the secondaries of theoutput transformer 22; and then the leakage inductances L and L1 of thetransformer itself will function in the same way previously described inconnection with the cushioning operation, and the additional inductancesXL and XL1 may be omitted.

In applying the principles and practices of this invention to thecircuit organization of Fig. 3, cushioning capacitors C and Cl andassociated resistors R and R1 are connected directly across the cathodeand anode of the respective tubes T and T1 in the same way and for thesame purpose as described in connection with Fig. 1.

Since the theories and mode of operation relating to the parallel typeinverter are generally familiar to those skilled in the art, it is notnecessary to discuss this operation in detail, but merely point out theconditions under which the cushioning means of this invention acts tr.prolong the life of the tubes. Starting with the time indicated at a inFigs. 1A-4C, when the tube T fires, this tube becomes conducting andallows current to flow from the direct current source through thecorresponding primary 2l of the output transformer 22, as indicated bythe current curve in Fig. 4A, this current rising at a limited rate dueto the leakage inductance L of the transformer and the additionalinductance XL, and also decreasing from a peak value under ordinaryoperating conditions, primarily due to the influence of the load. Theparticular shape of the curve of conduction current through a tube whenit i'lres varies with the character of the load on the output circuitand other factors, and the curves shown in Figs. 4A-4C are merelyrepresentative of the character of current ow ordinarily found withconventional loads of a l non-inductive character.

When the tube T starts to conduct at the time a, the potential betweenits anode and cathode drops to the relatively low voltage of its arcdrop, as indicated in Fig. 4C. Also, when the tube T fires, a charging-current flows through the other primary 2l1 of the output transformer22 to the commutating condenser CC, as indicated in Fig. 4B.

Considering now the critical time indicated at b in Figs. iA-4C when theother tube Tl res, the commutating condenser CC performs its function ofstopping conduction through the tube T previously ilred by causing itsanode to suddenly assume a negative potential with respect to itscathode, as indicated in the curve of Fig. 4B. In the operation of thistype of parallel inverter, the

` condenser CC is arranged to maintain the anode of the tube T lastfired at a negative potential long enough for effective deionization topermit the grid of this tube to regain control; and in manyvapplicationsand under the various load conditions, the negative potential providedby the commutating condenser CC is relatively high. In other words, aninverse voltage provided by the commutating condenser CC is suddenlyapplied to the tube T last ilred, While it is still conductingapproximately full load current; and this sudden application of a highinverse .voltage adversely affects the life of the tube for the samereasons previously explained.

When the cushioning means of this invention is employed, however, whenthe inverse voltage is applied to the tube T by the commutatingcondenser CC at the time b, the cushioning capacitor C associated withthis tube T is substantially discharged through the relatively lowresistance of the arc drop of this tube; and consequently the actualpotential between the anode and cathode of the tube T, instead of risingabruptly to the full voltage provided by the CQntlOl condenser CC asindicated at 20 in Fig. 4C. is restrained by the action of thecushioning capacitor C to rise at a -controlled rate, as indicated at 21in Fig. 4C. In this connection, the additional inductance XL retards thegrowth of charging current to the cushioning capacitor C; and althoughthis cushioning capacitor C retards the rate oi' increase in thenegative potential on the anode of the tube T, this potential is stilleii'ective t0 cause deionization of the tube T and permit its grid toregain control. The capacity of the cushioning capacitor C is relativelysmall as compared with that of the control condenser CC, and the timeinterval indicated as td in Fig. 4C during which the anode of the tube Tis negative is not materially effected by the use of the cushioning.capacitor C.

v ployed in various other applications of gas discharge tubes torectifying organizations. In view of the foregoing explanation, it isapparent that deleterious sputtering of anode material and the need forsome form of cushioning means arises whenever the ring time of the tube,inductive character of the load, or other conditions in the circuitorganization.. result in the application of a high inverse voltage whenconduction ceases: and it can be appreciated that the cushioning meansof this invention may be advantageously employed in any situation wherethese conditions exist. In the case of a high voltage high-powerpolyphase rectier, for example, even though the tiring time of the tubesmay not be delayed by grid control, the period of commutation.corresponding to the time t0 to tl in Figs. 2A to 2D, may be.approximately 20 to 30 electrical degrees, due to the relatively highleakage reactance load of the associated transformer, togetherwith thereactance of the load or other parts of the circuit organization; andduring this commutation time the voltages between phases will have timeto increase from zero and apply .an inverse voltage to the tube lastconducting in excess of the tolerable inverse voltage, and therebycausing damaging sputtering of anode material. For instance, in athree-phase rectifier organization for a 3000voltage system, the voltagebetween the phases during a commutation period of twenty electricaldegrees will increase from 0 to some 2000 volts, so that without the useof a cushioning means in accordance with this invention. the inversevoltage applied when conduction ceases is high enough to causedeleterious ionic bombardment of the anode. Even though the gas fillingis mercury vapor and the gas disappearance is not a factor in the lifeof the tube, such excessive ionic bombardment of the anode materiallyaffects the ability of the tube to .withstand inverse voltage; and it isfound that the use of the cushioning means of this invention serves toraise the tolerable inverse voltage rating for rectiilers of themercury-vapor type.

Other specific instances of the application and utility of thecushioning means of this invention might be discussed; but it isbelieved that the foregoing will indicate the general scope o! useandapplication of the invention.

amano 'Ihe specific circuit organizations and method of cushioningillustrated and described as characteristic of the invention, togetherwith the various explanatory graphs and theoretical discussion of thephenomena assumed to be involved in connection with the purpose of theinvention and the results desired, are merely typical or' illustrativeof the nature and character of the invention; and I desire to have itunderstood that my invention is in no sense limited to the specificarrangement-s of parts and circuits shown and described, or to theparticular procedures and processes discussed, and in particular is notto be assumed as necessarily involving the particular theories andassumptions herein given in explanation of tube performance, mode ofoperation, and results obtained by practicing the invention.

What I claim is:

i. In a rectifying circuit organization of the character described, incombination with a plurality of gas discharge rectifying tubes having agas iilling adversely affected by excessive ionic bombardment of theanode, a full-wave rectifying circuit organization including a source ofalternating current and an inductive direct current circuit connected tothe anode circuits of said tubes. said circuit organization acting attimes to subject said tubes to a relatively high inverse voltageimmediately after conduction ceases, of a capacitor and a resistorconnected in series across the anode and cathode of each tube and actingto control the rate' of rise of potential difference between the cathodeand the anode caused by the application of such inverse voltage, saidcapacitors and resistors having values to avoid an oscillatorycondition,whereby the damaging sputtering of anode material by ionicv bombardmentof the anode resulting from such diiference oi potential is kept withincontrolled iimits.

2. A rectifying organization comprising in combination. a plurality ofgrid lcontrolled gas discharge rectiiying tubes connected to a highlyinductive load, means for governing the lring'o said tubes forcontrollable intervals and acting to provide commutation periods ofsimultaneous conduction by two tubes, transformer windings providingenergization of the anocles of said tubes with alternating potentials ofopposite instantaneous polarities and acting to apply across each tubeat times a relatively high inverse voltage as soon as conduction ceases,and a capacitor and a resistor connected in series across the anodeanclcathode oi each of said tubes to retard the rate of rise inpotential between the cathode and the anode of that tube as the inversevoltage is applied thereto, said resistors having a value to provide adamped non-oscillatory circuit through the tubes, whereby the damagingsputtering of the anode material is limited.

3. In power conversion apparatus of the character described employinggrid controlled gas tubes, the combination with a tube having an inertgas iilling subject to clean-up by excessive ionic bombardment of theanode, a circuit organization including said tube and a source ofalternating voltage, said circuit organization including a relativelylarge inductance in the anode circuit of said tube and acting at timesto apply across the anode and cathode of said tube immediately afterconduction of full load current an inverse voltage much higher than said14 rial to reduce the life of the tube, and non-oscillatory circuitmeans including a capacitor and a resistor in series associated with thecathode and l anode of said tube for controlling the rate o1' rise l o!the actual potential between the cathode and 'titi :said alternatingcurrent circuit,

the anode'as said inverse voltage is applied.

4. Power conversion apparatus of the character described comprising incombination, a plurality of grid control gaseous discharge tubes havinginert gas fillings and subject to gas clean-up by excessive ionicbombardment of the anodes of said tubes, a circuit organizationproviding fullwave rectifying connections between an alternatingcurrentcircuit and an inductive direct current circuit, said circuitorganization render- 'have a high diilerence in potential, and therebyapply across the anode and cathode of a tube immediately followingconduction of full load current a high inverse voltage exceeding themaximum instantaneous value of the voltage in cushioning means includingin series a capacitor and resistor for automatically limiting the rateof rise in potential between said anode and cathode of each tube uponapplication of an inverse voltage in conformity with the characteristicsof the tube and the intensity of the current conducted, and therebycause a rate of deionization as compared with the increase in thevoltage accelerating the positive gas ions toward the cathode which willobviate excessive sputtering of the anode material by ionic bombardment,said cushioning means including damping means for avoiding anoscillatory condition in the circuit through said tube 5. A circuitorganization for electron discharge tubes comprising, two gridcontrolled gas discharge tubes each having a thermionic emissive cathodeandv a gas lling, the pressure of said gas nlling being adverselyaiected by excessive ionic bombardment of the anode, afull-wavelectifying circuit organization including a relatively largeinductance in a direct current circuit and other inductance in the anodecircuit of each tube, grid control means for rendering said tubesconductive to supply current to said inductance at different intervalswith a commutation period of concurrent conduction, said circuitorganiza-l tion under certain conditions of said grid control meansretarding the ring of said tubes for a Isubstantial angle acting tosubject the anodes of said tubes to a relatively high inverse voltageimmediately after full conduction ceases, and circuit means including acapacitor and resistor in series associated with each tube forarticially and automatically retarding the rate of rise of the potentialdifference between its anode and cathode as the inverse voltage isapplied thereto.

6. In a rectifying organization, the combination with a'pair of gridcontrol gas tubes having a gas lling adversely affected by excessiveionic 'bombardment of the anode, a rectifying circuit organizationincluding said tubes and a highly inductive load, said circuitorganization applyrent between said tubes at a time when said al-gternating potentials are near their maximum instantaneous values,thereby subjecting each tube to a high inverse voltage immediately aftercon-A duction ceases, of a capacitor and resistor conl nected in seriesvacross said cathode and anode to retard the rate of rise in potentialdifference between said cathode and anode, and thereby control theamount of sputtering of anode material by the impact of positive gasions remaining from the immediately preceding conduction under theinfluence of said potential diierence, said capacitor and resistorhaving values to prevent oscillatory conditions in the circuit throughthe tube.

7. In a rectifying organization of the character described, thecombination with a gas discharge tube having a gas illled envelopeincluding a heated electron emissivecathode and an anode, a circuitorganization including a source of alternating current and an inductiveload associated with said tube to act at times to subject said tube to ahigh inverse voltage immediately after conduction ceases, and acapacitor and a resistor connected directly in series across the anodeand cathode of said tube, said capacitor acting to retard the rate ofrise o1' the potential between the anode and cathode caused by theapplication of the inverse voltage and thereby limit the sputtering ofanode material by the application of such inverse voltage, said resistorlimiting the magnitude of current through the tube in its conductingdirection upon discharge of said capacitor and also providing damping toprevent an oscillatory condition in the circuit through the tube.

8. In a rectifying organization of the character described, thecombination with a source of alternating voltages having differentinstantaneous values at the same time and ahighly inductive directcurrent circuit, of a. plurality of grid controlled gas discharge tubesalternately red at such points with respect to the alternating voltagesas to cause a much higher inverse voltage to be applied to each tubeimmediately after conduction through that tube ceases, andnon-oscillatory cushioning means including a capacitor and resistor inseries associated with each of said tubes to retard the rate of rise ofthe potential between the anode and cathode of that tube uponapplication of said 'inverse voltage thereto, whereby sputtering ofanode material by ionic bom- -bardment resulting from the inversepotential as between the cathode and anode of the tubes is restricted inspite of the high inverse voltage applied to the tubes. I

9. In a rectifying organization, the combination with a source ofalternating current and an inductive load, of a circuit organizationincluding a plurality of grid controlled gas discharge tubes forsupplying unidirectional current pulses from said source to said load,said circuit organization sometimes acting to retard the conductionthrough said tubes for a substantial angle and thereby subject each ofsaid tubes to a relatively high inverse voltage immediately followingconduction, and non-oscillatory cushioning means ascociated `with eachof said tubes for controlling the rate of rise of the potential betweenthe cathode and anode of that tube upon application of the inversevoltage thereto, said cushioning means including a resistor conductingcharging current to a capacitor to provide across the tube a potentialdifference increasing at a restricted rate, whereby the rate of rise ofthe potential as between the cathode and anode of the tube as comparedwith the deionization characteristics of the tube is limited to restrictthe ionic bombardment of the cathode and avoid damaging sputtering cithe anode material.

l0. A circuit organization for grid controlled gas discharge tubescomprising, two grid controlled gas i-llled tubes having their anodesconn nected to the opposite terminals of a transformer winding having amid-tap connected in series with a relatively large inductance to thecatia odes of said tubes, means for governing the potential on the gridsof said tubes to cause them 4 to be conductive alternately atpredetermined intervals with a brief commutation period of simultaneousconduction, said grid control means acting under `someoperating,conditions to retard the iiring of said tubes for asubstantial angle and thereby cause the anode of each tube to have arelatively high inverse voltage applied thereto immediately after thecommutation period and cessation of conduction through that tube, and acapacitor and resistor connected in series directly across the anode andcathode of each tube for limiting the rate of rise of the difierence ofpotential between the anode and cathode of the corresponding tube as theinverse voltage is applied to the anode of that tube, said reslstors andcapacitors having values to prevent an oscillatory condition in thecircuits including said tubes.

11. A circuit organization for supplying rectied alternating current toan inductive load comprising, two grid controlled gas discharge tubeshaving their anode circuits connected to the two halves of the secondaryof a transformer, means for controlling the grids of said tubes torender them conductive alternately to supply current from said twohalves of said transformer to an inductive load associated with saidtransformer, said load acting tomaintain conduction through each tube inturn for a time after the voltage applied to its anode from theassociated secondary of said transformer has become negative, saidcircuit organization acting to apply to the anode of each tube arelatively high inverse voltage immediately after conduction throughthat tube ceases, and a capacitor and a resistor for each tube directlyconnected in series across the anode and cathode of that tube to limitthe rate of rise of potential between the cathode and anode of the tubeupon the application of said inverse voltage, and thereby limit theionic bombardment of the anode, said resistors having a value to dampout oscillations in the circuits including said tubes.

12. A circuit organization of the character described comprising, twogrid controlled gas discharge tubes having their anode circuitsconnected to the two halves of a transformer winding, said transformerwinding having substantial leakage reactance. means for governing thepotentials on the grids of said tubes to render them conductivealternately at predetermined intervals, a direct current circuitincluding a relatively large inductance associated with said transformerwinding and the anode circuits of said tubes, said inductance and saidgrid control means cooperating at times to maintain conduction througheach tube in turn until the voltages in the two halves of saidtransformer winding have nearly obtained their maximum instantaneousvalues of opposite polarities and thereby cause the anode of each tubeto have a relatively high inverse voltage applied thereto upon cessationof conduction of that tube, and non-oscillatory cushioning meansincluding a capacitor and resistor directly connected in series across17 the anode and cathode of each tube for automatically limiting therate of rise of potential of the anode of each tube in turn with respectto its cathode as said inverse voltage is applied to said anode.

13. A rectifier circuit organization comprising in combination, aplurality of grid control 'gas tubes having their operatingcharacteristics adversely affected by excessive ionic bambardment of theanode, a circuit organization includingadirect current circuit and analternating current circuit associated with the anode circuits of saidtubes, said organization including means for controlling the ring ofsaid tubes and acting at times to cause the ring of the tubes at timeswhen a relatively high inverse voltage is applied across each tube inturn immediately after that tube has conducted full load current, andcircuit means associated witheach tube for artificially andautomatically controlling the rate of rise of potential between theanode and cathode of that tube upon the application of said relativelyhigh inverse voltage, said circuit means including a capacitor andresistor in series directly connected across the anode and cathode ofthe tube and having values preventing an oscillatory condition in thecircuit through said tube, whereby ionic bombardment of said tubes .bysuch high inverse voltages is kept within tolerable limits.

14. A circuit organization of the character described comprising incombination, a source of alternating voltages out of phase and havingdifferent instantaneous values at a given time, a direct current circuitincluding a relatively large inductance, a plurality of grid control gaslled tubes having anode circuits associated with said direct currentcircuit and alternating voltages, the operating characteristics of saidtubes being 18 adversely affected by excessive ionic bombardment oftheir anodes, each of said tubes under certain operating conditionsbeing subjected to an inverse voltage much higher than the maximuminstantaneous values of said alternating voltages immediately afterconduction of full current by that tube, and a capacitor and resistor inseries directly connected across each tube and having values dependentupon the deionization characteristics of the tubes, said capacitor andresistor cooperating to retard-the rate of rise of potential between theanode and cathode of the associated tube upon application of saidinverse voltage and thereby keep the ionic bombardment of the anode ofsaid tube within tolerable limits, said resistor limiting the magnitudeof current through the associated tube in its conducting direction upondischarge of said capacitor and also providing damping to preventoscillation in the anode circuit of the tube.

DONALD V. EDWARDS.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,999,597 Rudenberg Apr. 30, 19351,999,736 Morrison Apr. 30, 1935 2,097,578 Swart et al, Nov. 2, 19372,113,163 Schlesinger Apr. 5, 1938 2,125,799 Metcalf Aug. 2, 19382,148,578 Pullis Feb. 28, 1939 2,234,690 Depp Mar. 11, 1941 2,250,819Wolf July 29, 1941 2,394,535 Dawson Feb. 12, 1946

